Moloney leukemia virus synthesizes proviral DNA by reverse transcription several hours after virion penetration of a susceptible cell. The proviral DNA of Moloney leukemia virus is being studied using nucleic acid hybridization and transfection techniques. The proviral DNA is being characterized in three molecular states. 1) A closed, circular form of 6 x 10 to the 6th power daltons mass is found in small amounts 9 hours after infection. This DNA is infectious by one-hit kinetics and therefore contains the entire genetic information of the virus. Present work is concentrated on developing a restriction enzyme map of this closed, circular form. 2) A non-closed, circular DNA form is present in larger amounts after infection. This material represents a heterogeneous collection of proviral DNA with many partially single-stranded molecules which have not been completed. Only a small fraction of these molecules are infectious, and these are those which are almost completely double-stranded. Present work is concentrated on isolating this infectious form and determining whether it is linear or circular. This DNA is regarded as the immediate precursor to the closed, circular DNA. 3) The third form is the integrated form of the proviral DNA. The viral genome in this integrated state is presently being studied by restriction enzyme analysis and its restriction enzyme map is being compared with that of the closed, circular DNA. BIBLIOGRAPHIC REFERENCES: Gianni, A.M., J.R. Hutton, D. Smotkin and R.A. Weinberg (1976) Proviral DNA of Moloney Leukemia Virus: Purification and Visualization. Science, in press.